Transmutation - external condition to internal process

How do the desired biological changes of an animal coupled with the circumstances of its environment translate the desired change to the chromosomes? I understand that if an desired a change, such as longer arms, different color fur, or different shaped beak, that the process takes place in the chromosomes, much in the same manner as food in the oven. It’s a matter of time and temperature that created the desired effect. But what I don’t understand is how that desired effect is transmitted to the chromosomes.
I saw this example on a program about evolution, but it never bridged the gap for me.
A group of field mice from larger group of field mice, all having brown fur coats migrate to a mountain with black rocks. Over time the fur of the mice on the mountain mutated from brown to black, thus an evolutionary change, since blending in with the surrounding rocks gave better protection from predators.
How does a group of brown mice, just but looking at the surrounding rocks, transmit the desirous change (black fur) to the chromosomes? Is it transmitted through the eyes, the animal sees or thinks the desired change and thereby turns on the “oven” to the precise time and temperature? Once the transmission is made, does the changed chromosome just remain dormant in the animal only to be expressed in the next generation? Or do the mice wittingly or unwittingly seek mates already with an expressed change (darker fur, but not black yet) to mate with to create a dominant gene in the next generation? It seems to me that all this would be a seeking out of subtle changes in mates to create dominant genes with the desired effects.